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Spring-back phenomena are critical in stamping procedures for advanced high strength steel. An analytical model is developed to predict the spring-back effect for a U-channel part with post-stretching process. The stress distribution is obtained by direct application of material constitutive relationship. The subjected loading conditions are sequentially bending, (un-bending), and uniform stretching, based on different zones in the part. Both the loading history and the friction effects are considered in the model. The bending moments are obtained to generate a theoretical spring-back shape. Great performance in spring-back control is achieved by applying certain high level of external forces. FE simulation is conducted for the identical stamping process with post-stretching. Good correlation is observed between the analytical and numerical solutions/experimental results under various scenarios.

Dimensional accuracy of punched hole is an essential consideration for high-quality sheet metal forming. An out-of-shape hole can give rise to manufacturing issues in the subsequent production processes thus inducing quality defects on a vehicle body. To understand the effects of punch shapes and cutting configurations on punched hole diameter deviations, a systematical experimental study was conducted for multiple types of AHSS (DP1180, DP980, DP590) and one mild steel. Flat, conical and rooftop punches were tested respectively with three cutting clearances on each material. The measurement results indicated different diameter enlargement modes based on the punch profiles, and dimensional discrepancies were found to be more significant with the stronger materials and higher cutting clearance. To uncover the mechanism of punched hole enlargement, a series of finite element simulations were established for numerical investigation.

Twist spring-back would interfere with stamping or assembling procedures for advanced high strength steel. A “homeopathic” resolution for controlling the twist spring-back is proposed using unbalanced post-stretching configuration. Finite element forming simulation is applied to evaluate and compare the performance for each set of unbalanced post-stretching setup. The post-stretching is effectuated by stake bead application. The beads are separated into multiple independent segments, the height and radii of which can be adjusted individually and asymmetrically. Simulation results indicate that the twist spring-back can be effectively controlled by reducing the post-stretching proximate to the asymmetric part area. Its mechanism is qualitatively revealed by stress analyses, that an additional but acceptable cross-sectional spring-back re-balances the sprung asymmetrical geometry to counter the twist effect.

Multiple hybrid bead designs were investigated in this study to control the springback on DP780 samples using post-stretching technique. The performance of the four different hybrid bead designs was evaluated by measuring the minimum blank-lock tonnage required to control the springback during a U-channel stamping process. A finite element (FE) model of the U-channel stamping process was developed to simulate the process and predict the minimum blank-lock tonnage required for springback control using each of the hybrid bead designs. It is shown that the developed FE model predicts both the required minimum blank-lock tonnage for post-stretching, and the springback profile, with good accuracy.

At the dawn of battery electric vehicles (BEVs), protection of automotive battery systems as well as passengers, especially from severe side impact, has become one of the latest and most challenging topics in the BEV crashworthiness designs. Accordingly, two material-selection concepts are being justified by the automotive industry: either heavy-gauge extruded aluminum alloys or light-gauge advanced high-strength steels (AHSSs) shall be the optimal materials to fabricate the reinforcement structures to satisfy both the safety and lightweight requirements. In the meantime, such a justification also motivated an ongoing C-STARTM (Cliffs Steel Tube as Reinforcement) Protection project, in which a series of modularized steel tube assemblies, were demonstrated to be more cost-efficient, sustainable, design-flexible, and manufacturable than the equivalent extruded aluminum alloy beams as BEV reinforcement structures.